Most organic molecules and functional groups are transparent in the portions of the electromagnetic ULTRAVIOLET SPECTROSCOPY

Transcription

1 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page 381 A P T E R 7 ULTRAVILET SPETRSPY Most organic molecules and functional groups are transparent in the portions of the electromagnetic spectrum that we call the ultraviolet (UV) and visible (VIS) regions that is, the regions where wavelengths range from 190 nm to 800 nm. onsequently, absorption spectroscopy is of limited utility in this range of wavelengths. owever, in some cases we can derive useful information from these regions of the spectrum. That information, when combined with the detail provided by infrared and nuclear magnetic resonance (NMR) spectra, can lead to valuable structural proposals. 7.1 TE NATURE F ELETRNI EXITATINS When continuous radiation passes through a transparent material, a portion of the radiation may be absorbed. If that occurs, the residual radiation, when it is passed through a prism, yields a spectrum with gaps in it, called an absorption spectrum. As a result of energy absorption, atoms or molecules pass from a state of low energy (the initial, or ground state) to a state of higher energy (the excited state). Figure 7.1 depicts this excitation process, which is quantized. The electromagnetic radiation that is absorbed has energy exactly equal to the energy difference between the excited and ground states. In the case of ultraviolet and visible spectroscopy, the transitions that result in the absorption of electromagnetic radiation in this region of the spectrum are transitions between electronic energy levels. As a molecule absorbs energy, an electron is promoted from an occupied orbital to an unoccupied orbital of greater potential energy. Generally, the most probable transition is from the highest occupied molecular orbital (M) to the lowest unoccupied molecular orbital (LUM). The energy differences between electronic levels in most molecules vary from 125 to 650 kj/mole (kilojoules per mole). For most molecules, the lowest-energy occupied molecular orbitals are the s orbitals, which correspond to s bonds. The p orbitals lie at somewhat higher energy levels, and orbitals that hold unshared pairs, the nonbonding (n) orbitals, lie at even higher energies. The unoccupied, or antibonding orbitals (p * and s *), are the orbitals of highest energy. Figure 7.2a shows a typical progression of electronic energy levels. E(excited) ΔE = [E(excited) E(ground)] = hυ FIGURE 7.1 The excitation process. E(ground) 381

2 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Ultraviolet Spectroscopy σ* σ* π* π* n π* Unoccupied levels Energy In all compounds other than alkanes, the electrons may undergo several possible transitions of different energies. Some of the most important transitions are U Increasing energy (a) s U s * s U p* p U p* n U s* n U p* n σ* FIGURE 7.2 Electronic energy levels and transitions. n π σ In alkanes In carbonyl compounds In alkenes, carbonyl compounds, alkynes, azo compounds, and so on In oxygen, nitrogen, sulfur, and halogen compounds In carbonyl compounds ccupied levels Figure 7.2b illustrates these transitions. Electronic energy levels in aromatic molecules are more complicated than the ones depicted here. Section 7.14 will describe the electronic transitions of aromatic compounds. learly, the energy required to bring about transitions from the highest occupied energy level (M) in the ground state to the lowest unoccupied energy level (LUM) is less than the energy required to bring about a transition from a lower occupied energy level. Thus, in Figure 7.2b an n U p* transition would have a lower energy than a p U p* transition. For many purposes, the transition of lowest energy is the most important. Not all of the transitions that at first sight appear possible are observed. ertain restrictions, called selection rules, must be considered. ne important selection rule states that transitions that involve a change in the spin quantum number of an electron during the transition are not allowed to take place; they are called forbidden transitions. ther selection rules deal with the numbers of electrons that may be excited at one time, with symmetry properties of the molecule and of the electronic states, and with other factors that need not be discussed here. Transitions that are formally forbidden by the selection rules are often not observed. owever, theoretical treatments are rather approximate, and in certain cases forbidden transitions are observed, although the intensity of the absorption tends to be much lower than for transitions that are allowed by the selection rules. The n U p* transition is the most common type of forbidden transition. n π σ π π* (b) σ π* σ σ*

3 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Principles of Absorption Spectroscopy TE RIGIN F UV BAND STRUTURE For an atom that absorbs in the ultraviolet, the absorption spectrum sometimes consists of very sharp lines, as would be expected for a quantized process occurring between two discrete energy levels. For molecules, however, the UV absorption usually occurs over a wide range of wavelengths because molecules (as opposed to atoms) normally have many excited modes of vibration and rotation at room temperature. In fact, the vibration of molecules cannot be completely frozen out even at absolute zero. onsequently, a collection of molecules generally has its members in many states of vibrational and rotational excitation. The energy levels for these states are quite closely spaced, corresponding to energy differences considerably smaller than those of electronic levels. The rotational and vibrational levels are thus superimposed on the electronic levels. A molecule may therefore undergo electronic and vibrationalrotational excitation simultaneously, as shown in Figure 7.3. Because there are so many possible transitions, each differing from the others by only a slight amount, each electronic transition consists of a vast number of lines spaced so closely that the spectrophotometer cannot resolve them. Rather, the instrument traces an envelope over the entire pattern. What is observed from these types of combined transitions is that the UV spectrum of a molecule usually consists of a broad band of absorption centered near the wavelength of the major transition. 7.3 PRINIPLES F ABSRPTIN SPETRSPY The greater the number of molecules capable of absorbing light of a given wavelength, the greater the extent of light absorption. Furthermore, the more effectively a molecule absorbs light of a given wavelength, the greater the extent of light absorption. From these guiding ideas, the following empirical expression, known as the BeerLambert Law, may be formulated. E 1 v 3 v 2 v 1 v 4 Vibrational levels ELETRNI EXITED STATE E 0 v 3 v 2 v 1 Vibrational levels ELETRNI GRUND STATE FIGURE 7.3 Electronic transitions with vibrational transitions superimposed. (Rotational levels, which are very closely spaced within the vibrational levels, are omitted for clarity.)

4 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Ultraviolet Spectroscopy A = log(i 0 /I ) = ecl for a given wavelength Equation 7.1 A = absorbance I 0 = intensity of light incident upon sample cell I = intensity of light leaving sample cell c = molar concentration of solute l = length of sample cell (cm) e = molar absorptivity The term log (I 0 /I ) is also known as the absorbance (or the optical density in older literature) and may be represented by A. The molar absorptivity (formerly known as the molar extinction coefficient) is a property of the molecule undergoing an electronic transition and is not a function of the variable parameters involved in preparing a solution. The size of the absorbing system and the probability that the electronic transition will take place control the absorptivity, which ranges from 0 to Values above 10 4 are termed high-intensity absorptions, while values below 10 3 are low-intensity absorptions. Forbidden transitions (see Section 7.1) have absorptivities in the range from 0 to The BeerLambert Law is rigorously obeyed when a single species gives rise to the observed absorption. The law may not be obeyed, however, when different forms of the absorbing molecule are in equilibrium, when solute and solvent form complexes through some sort of association, when thermal equilibrium exists between the ground electronic state and a low-lying excited state, or when fluorescent compounds or compounds changed by irradiation are present. 7.4 INSTRUMENTATIN The typical ultravioletvisible spectrophotometer consists of a light source, a monochromator, and a detector. The light source is usually a deuterium lamp, which emits electromagnetic radiation in the ultraviolet region of the spectrum. A second light source, a tungsten lamp, is used for wavelengths in the visible region of the spectrum. The monochromator is a diffraction grating; its role is to spread the beam of light into its component wavelengths. A system of slits focuses the desired wavelength on the sample cell. The light that passes through the sample cell reaches the detector, which records the intensity of the transmitted light I. The detector is generally a photomultiplier tube, although in modern instruments photodiodes are also used. In a typical double-beam instrument, the light emanating from the light source is split into two beams, the sample beam and the reference beam. When there is no sample cell in the reference beam, the detected light is taken to be equal to the intensity of light entering the sample I 0. The sample cell must be constructed of a material that is transparent to the electromagnetic radiation being used in the experiment. For spectra in the visible range of the spectrum, cells composed of glass or plastic are generally suitable. For measurements in the ultraviolet region of the spectrum, however, glass and plastic cannot be used because they absorb ultraviolet radiation. Instead, cells made of quartz must be used since quartz does not absorb radiation in this region. The instrument design just described is quite suitable for measurement at only one wavelength. If a complete spectrum is desired, this type of instrument has some deficiencies. A mechanical system is required to rotate the monochromator and provide a scan of all desired wavelengths. This type of system operates slowly, and therefore considerable time is required to record a spectrum. A modern improvement on the traditional spectrophotometer is the diode-array spectrophotometer. A diode array consists of a series of photodiode detectors positioned side by side on a silicon crystal. Each diode is designed to record a narrow band of the spectrum. The diodes are connected so that the entire spectrum is recorded at once. This type of detector has no moving parts and

5 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Presentation of Spectra 385 can record spectra very quickly. Furthermore, its output can be passed to a computer, which can process the information and provide a variety of useful output formats. Since the number of photodiodes is limited, the speed and convenience described here are obtained at some small cost in resolution. For many applications, however, the advantages of this type of instrument outweigh the loss of resolution. 7.5 PRESENTATIN F SPETRA The ultravioletvisible spectrum is generally recorded as a plot of absorbance versus wavelength. It is customary to then replot the data with either e or log e plotted on the ordinate and wavelength plotted on the abscissa. Figure 7.4, the spectrum of benzoic acid, is typical of the manner in which spectra are displayed. owever, very few electronic spectra are reproduced in the scientific literature; most are described by indications of the wavelength maxima and absorptivities of the principal absorption peaks. For benzoic acid, a typical description might be l max = 230 nm log e = Figure 7.4 is the actual spectrum that corresponds to these data. FIGURE 7.4 Ultraviolet spectrum of benzoic acid in cyclohexane. (From Friedel, R. A., and M. rchin, Ultraviolet Spectra of Aromatic ompounds, John Wiley and Sons, New York, Reprinted by permission.)

6 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Ultraviolet Spectroscopy 7.6 SLVENTS The choice of the solvent to be used in ultraviolet spectroscopy is quite important. The first criterion for a good solvent is that it should not absorb ultraviolet radiation in the same region as the substance whose spectrum is being determined. Usually solvents that do not contain conjugated systems are most suitable for this purpose, although they vary regarding the shortest wavelength at which they remain transparent to ultraviolet radiation. Table 7.1 lists some common ultraviolet spectroscopy solvents and their cutoff points or minimum regions of transparency. f the solvents listed in Table 7.1, water, 95% ethanol, and hexane are most commonly used. Each is transparent in the regions of the ultraviolet spectrum in which interesting absorption peaks from sample molecules are likely to occur. A second criterion for a good solvent is its effect on the fine structure of an absorption band. Figure 7.5 illustrates the effects of polar and nonpolar solvents on an absorption band. A nonpolar solvent does not hydrogen bond with the solute, and the spectrum of the solute closely approximates the spectrum that would be produced in the gaseous state, in which fine structure is often observed. In a polar solvent, the hydrogen bonding forms a solutesolvent complex, and the fine structure may disappear. TABLE 7.1 SLVENT UTFFS Acetonitrile 190 nm n-exane 201 nm hloroform 240 Methanol 205 yclohexane 195 Isooctane 195 1,4-Dioxane 215 Water % Ethanol 205 Trimethyl phosphate 210 FIGURE 7.5 Ultraviolet spectra of phenol in ethanol and in isooctane. (From oggeshall, N. D., and E. M. Lang, Journal of the American hemical Society, 70 (1948): Reprinted by permission.)

7 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page What is a hromophore? 387 TABLE 7.2 SLVENT SIFTS N TE n U p * TRANSITIN F AETNE Solvent l l max (nm) A third criterion for a good solvent is its ability to influence the wavelength of ultraviolet light that will be absorbed via stabilization of either the ground or the excited state. Polar solvents do not form hydrogen bonds as readily with the excited states of polar molecules as with their ground states, and these polar solvents increase the energies of electronic transitions in the molecules. Polar solvents shift transitions of the n U p * type to shorter wavelengths. n the other hand, in some cases the excited states may form stronger hydrogen bonds than the corresponding ground states. In such a case, a polar solvent shifts an absorption to longer wavelength since the energy of the electronic transition is decreased. Polar solvents shift transitions of the p U p * type to longer wavelengths. Table 7.2 illustrates typical effects of a series of solvents on an electronic transition. 7.7 WAT IS A RMPRE? Although the absorption of ultraviolet radiation results from the excitation of electrons from ground to excited states, the nuclei that the electrons hold together in bonds play an important role in determining which wavelengths of radiation are absorbed. The nuclei determine the strength with which the electrons are bound and thus influence the energy spacing between ground and excited states. ence, the characteristic energy of a transition and the wavelength of radiation absorbed are properties of a group of atoms rather than of electrons themselves. The group of atoms producing such an absorption is called a chromophore. As structural changes occur in a chromophore, the exact energy and intensity of the absorption are expected to change accordingly. Very often, it is extremely difficult to predict from theory how the absorption will change as the structure of the chromophore is modified, and it is necessary to apply empirical working guides to predict such relationships. Alkanes. For molecules, such as alkanes, that contain nothing but single bonds and lack atoms with unshared electron pairs, the only electronic transitions possible are of the s U s * type. These transitions are of such a high energy that they absorb ultraviolet energy at very short wavelengths shorter than the wavelengths that are experimentally accessible using typical spectrophotometers. Figure 7.6 illustrates this type of transition. The excitation of the s-bonding electron to the s *-antibonding orbital is depicted at the right. Alcohols, Ethers, Amines, and Sulfur ompounds. In saturated molecules that contain atoms bearing nonbonding pairs of electrons, transitions of the n U s * type become important. They are also σ* σ σ* σ FIGURE 7.6 s U s * transition.

8 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Ultraviolet Spectroscopy N σ N * FIGURE 7.7 n U s* transition. N N n (sp 3 ) σ N rather high-energy transitions, but they do absorb radiation that lies within an experimentally accessible range. Alcohols and amines absorb in the range from 175 to 200 nm, while organic thiols and sulfides absorb between 200 and 220 nm. Most of the absorptions are below the cutoff points for the common solvents, so they are not observed in solution spectra. Figure 7.7 illustrates an n U s* transition for an amine. The excitation of the nonbonding electron to the antibonding orbital is shown at the right. Alkenes and Alkynes. With unsaturated molecules, p U p* transitions become possible. These transitions are of rather high energy as well, but their positions are sensitive to the presence of substitution, as will be clear later. Alkenes absorb around 175 nm, and alkynes absorb around 170 nm. Figure 7.8 shows this type of transition. arbonyl ompounds. Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n U p * transitions. These are perhaps the most interesting and most studied transitions, particularly among carbonyl compounds. These transitions are also rather sensitive to substitution on the chromophoric structure. The typical carbonyl compound undergoes an n U p * transition around 280 to 290 nm (e = 15). Most n U p * transitions are forbidden and hence are of low intensity. arbonyl compounds also have a p U p * transition at about 188 nm (e = 900). Figure 7.9 shows the n U p * and p U p * transitions of the carbonyl group. 1 π π* π* π N n σ* FIGURE 7.8 p U p* transition. 1 ontrary to what you might expect from simple theory, the oxygen atom of the carbonyl group is not sp 2 hybridized. Spectroscopists have shown that although the carbon atom is sp 2 hybridized, the hybridization of the oxygen atom more closely approximates sp.

9 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page What is a hromophore? 389 σ* n π* Table 7.3 lists typical absorptions of simple isolated chromophores. You may notice that these simple chromophores nearly all absorb at approximately the same wavelength (160 to 210 nm). The attachment of substituent groups in place of hydrogen on a basic chromophore structure changes the position and intensity of an absorption band of the chromophore. The substituent groups may not give rise to the absorption of the ultraviolet radiation themselves, but their presence modifies the absorption of the principal chromophore. Substituents that increase the intensity of the absorption, and possibly the wavelength, are called auxochromes. Typical auxochromes include methyl, hydroxyl, alkoxy, halogen, and amino groups. ther substituents may have any of four kinds of effects on the absorption: 1. Bathochromic shift (red shift) a shift to lower energy or longer wavelength. 2. ypsochromic shift (blue shift) a shift to higher energy or shorter wavelength. 3. yperchromic effect an increase in intensity. 4. ypochromic effect a decrease in intensity. π* n (p y ) π σ (sp 2 sp) n (sp) FIGURE 7.9 Electronic transitions of the carbonyl group. π π*

10 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page Ultraviolet Spectroscopy TABLE 7.3 TYPIAL ABSRPTINS F SIMPLE ISLATED RMPRES lass Transition l max (nm) log e lass Transition l max (nm) log e RI n U s * RIN 2 n U p * 271 <1.0 RIIR n U s * RI p U p * RIN 2 n U s * n U p * RIS n U s * R 2 p U p * R 2 JR 2 p U p * n U p * RIKIR p U p * R n U p * RIKN n U p * 160 <1.0 RR n U p * RINJNIR n U p * 340 <1.0 RN 2 n U p * TE EFFET F NJUGATIN ne of the best ways to bring about a bathochromic shift is to increase the extent of conjugation in a double-bonded system. In the presence of conjugated double bonds, the electronic energy levels of a chromophore move closer together. As a result, the energy required to produce a transition from an occupied electronic energy level to an unoccupied level decreases, and the wavelength of the light absorbed becomes longer. Figure 7.10 illustrates the bathochromic shift that is observed in a series of conjugated polyenes as the length of the conjugated chain is increased. onjugation of two chromophores not only results in a bathochromic shift but increases the intensity of the absorption. These two effects are of prime importance in the use and interpretation of electronic spectra of organic molecules because conjugation shifts the selective light absorption of isolated chromophores from a region of the spectrum that is not readily accessible to a region that FIGURE I(J) n I 3 ultraviolet spectra of dimethylpolyenes. (a) n = 3; (b) n = 4; (c) n = 5. (From Nayler, P., and M.. Whiting, Journal of the hemical Society (1955): 3042.)

11 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page The Effect of onjugation on Alkenes 391 TABLE 7.4 EFFET F NJUGATIN N ELETRNI TRANSITINS l max (nm) e Alkenes Ethylene ,000 1,3-Butadiene ,000 1,3,5-exatriene ,000 b-arotene (11 double bonds) ,000 Ketones Acetone p U p * n U p * Buten-2-one p U p * 213 7,100 n U p * is easily studied with commercially available spectrophotometers. The exact position and intensity of the absorption band of the conjugated system can be correlated with the extent of conjugation in the system. Table 7.4 illustrates the effect of conjugation on some typical electronic transitions. 7.9 TE EFFET F NJUGATIN N ALKENES The bathochromic shift that results from an increase in the length of a conjugated system implies that an increase in conjugation decreases the energy required for electronic excitation. This is true and can be explained most easily by the use of molecular orbital theory. According to molecular orbital (M) theory, the atomic p orbitals on each of the carbon atoms combine to make p molecular orbitals. For instance, in the case of ethylene (ethene), we have two atomic p orbitals, f 1 and f 2. From these two p orbitals we form two p molecular orbitals, y 1 and y 2 *, by taking linear combinations. The bonding orbital y 1 results from the addition of the wave functions of the two p orbitals, and the antibonding orbital y 2 * results from the subtraction of these two wave functions. The new bonding orbital, a molecular orbital, has an energy lower than that of either of the original p orbitals; likewise, the antibonding orbital has an elevated energy. Figure 7.11 illustrates this diagrammatically. Notice that two atomic orbitals were combined to build the molecular orbitals, and as a result, two molecular orbitals were formed. There were also two electrons, one in each of the atomic p orbitals. As a result of combination, the new p system contains two electrons. Because we fill the lower-energy orbitals first, these electrons end up in y 1, the bonding orbital, and they constitute a new p bond. Electronic transition in this system is a p U p * transition from y 1 to y 2 *. Now, moving from this simple two-orbital case, consider 1,3-butadiene, which has four atomic p orbitals that form its p system of two conjugated double bonds. Since we had four atomic orbitals with which to build, four molecular orbitals result. Figure 7.12 represents the orbitals of ethylene on the same energy scale as the new orbitals for the sake of comparison. Notice that the transition of lowest energy in 1,3-butadiene, y 2 U y 3 *, is a p U p* transition and that it has a lower energy than the corresponding transition in ethylene, y 1 U y 2 *. This result is general. As we increase the number of p orbitals making up the conjugated system, the transition from the highest occupied molecular orbital (M) to the lowest unoccupied molecular orbital (LUM) has

12 14782_07_h7_p pp2.qxd 2/2/08 1:22 AM Page 392 Antibonding orbital 2 * ( 2 * = φ 1 ± φ 2 ) Atomic orbital φ 1 φ 1 π π* 1 FIGURE 7.11 Formation of the molecular orbitals for ethylene. 2p orbitals 2 * 1 4 * 3 * π π* π π* 2 φ 2 Atomic orbital φ 2 Bonding orbital ( 1 = φ 1 + φ 2 ) 4p orbitals 1 ethylene 2 2 1,3-butadiene 2 2 FIGURE 7.12 A comparison of the molecular orbital energy levels and the energy of the p U p * transitions in ethylene and 1,3-butadiene.

13 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page The Effect of onjugation on Alkenes 393 progressively lower energy. The energy gap dividing the bonding and antibonding orbitals becomes progressively smaller with increasing conjugation. Figure 7.13 plots the molecular orbital energy levels of several conjugated polyenes of increasing chain length on a common energy scale. Arrows indicate the MLUM transitions. The increased conjugation shifts the observed wavelength of the absorption to higher values. In a qualitatively similar fashion, many auxochromes exert their bathochromic shifts by means of an extension of the length of the conjugated system. The strongest auxochromes invariably possess a pair of unshared electrons on the atom attached to the double-bond system. Resonance interaction of this lone pair with the double bond(s) increases the length of the conjugated system. As a result of this interaction, as just shown, the nonbonded electrons become part of the p system of molecular orbitals, increasing its length by one extra orbital. Figure 7.14 depicts this interaction for ethylene and an unspecified atom, B, with an unshared electron pair. owever, any of the typical auxochromic groups, I, IR, IX, or IN 2, could have been illustrated specifically. In the new system, the transition from the highest occupied orbital y 2 to the antibonding orbital y 3 * always has lower energy than the p U p* transition would have in the system without the interaction. Although M theory can explain this general result, it is beyond the scope of this book. FIGURE 7.13 A comparison of the p U p * energy gap in a series of polyenes of increasing chain length. E N E R G Y B B + Ethylene Butadiene exatriene ctatetraene

14 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy FIGURE 7.14 Energy relationships of the new molecular orbitals and the interacting p system and its auxochrome. In similar fashion, methyl groups also produce a bathochromic shift. owever, as methyl groups do not have unshared electrons, the interaction is thought to result from overlap of the I bonding orbitals with the p system as follows: This type of interaction is often called hyperconjugation. Its net effect is an extension of the p system TE WDWARDFIESER RULES FR DIENES π* π Ethylene In butadiene, two possible p U p * transitions can occur: y 2 U y 3 * and y 2 U y 4 *. We have already discussed the easily observable y 2 U y 3 * transition (see Fig. 7.12). The y 2 U y 4 * transition is not often observed, for two reasons. First, it lies near 175 nm for butadiene; second, it is a forbidden transition for the s-trans conformation of double bonds in butadiene. 2 3 * B B 1 Molecular orbitals of resonance system n Nonbonding electrons on B s-trans conformation ψ 4 ψ 3 ψ 2 ψ nm (forbidden) 230 nm (allowed) s-cis conformation ψ 4 ψ 3 ψ 2 ψ nm (allowed) 271 nm (allowed)

15 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page The WoodwardFieser Rules for Dienes 395 A transition at 175 nm lies below the cutoff points of the common solvents used to determine UV spectra (Table 7.1) and therefore is not easily detectable. Furthermore, the s-trans conformation is more favorable for butadiene than is the s-cis conformation. Therefore, the 175-nm band is not usually detected. In general, conjugated dienes exhibit an intense band (e = 20,000 to 26,000) in the region from 217 to 245 nm, owing to a p U p * transition. The position of this band appears to be quite insensitive to the nature of the solvent. Butadiene and many simple conjugated dienes exist in a planar s-trans conformation, as noted. Generally, alkyl substitution produces bathochromic shifts and hyperchromic effects. owever, with certain patterns of alkyl substitution, the wavelength increases but the intensity decreases. The 1,3-dialkylbutadienes possess too much crowding between alkyl groups to permit them to exist in the s-trans conformation. They convert, by rotation around the single bond, to an s-cis conformation, which absorbs at longer wavelengths but with lower intensity than the corresponding s-trans conformation. 3 In cyclic dienes, where the central bond is a part of the ring system, the diene chromophore is usually held rigidly in either the s-trans (transoid) or the s-cis (cisoid) orientation. Typical absorption spectra follow the expected pattern: omoannular diene (cisoid or s-cis) Less intense, ε = 5,00015,000 λ longer (273 nm) s-trans eteroannular diene (transoid or s-trans) More intense, ε = 12,00028,000 λ shorter (234 nm) By studying a vast number of dienes of each type, Woodward and Fieser devised an empirical correlation of structural variations that enables us to predict the wavelength at which a conjugated diene will absorb. Table 7.5 summarizes the rules. Following are a few sample applications of these rules. Notice that the pertinent parts of the structures are shown in bold face. s-cis

16 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page 396 TABLE 7.5 EMPIRIAL RULES FR DIENES omoannular (cisoid) eteroannular (transoid) Parent l = 253 nm l = 214 nm Increments for: Double-bond-extending conjugation Alkyl substituent or ring residue 5 5 Exocyclic double bond 5 5 Polar groupings: Transoid: 214 nm bserved: 217 nm Transoid: 214 nm Ring residues: 3 5 = 15 Exocyclic double bond: nm bserved: nm I IR 6 6 Il, IBr 5 5 INR Transoid: 214 nm Alkyl groups: 3 5 = nm bserved: Exocyclic double bond nm Transoid: 214 nm Ring residues: 3 5 = 15 Exocyclic double bond: 5 R: nm bserved: 3 Exocyclic double bond 241 nm

17 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page arbonyl ompounds; Enones 397 In this context, an exocyclic double bond is a double bond that lies outside a given ring. Notice that the exocyclic bond may lie within one ring even though it is outside another ring. ften, an exocyclic double bond will be found at a junction point on rings. ere is an example of a compound with the exocyclic double bonds labeled with asterisks: 3 isoid: 253 nm Alkyl substituent: 5 Ring residues: 3 5 = 15 Exocyclic double bond: nm bserved: 3 * 3 3 * * nm 7.11 ARBNYL MPUNDS; ENNES R Three exocyclic double bonds = 3 5 = 15 nm 3 isoid: 253 nm Ring residues: 5 5 = 25 Double-bond-extending conjugation: 2 30 = 60 Exocyclic double bond: 3 5 = 15 3 : nm bserved: nm As discussed in Section 7.7, carbonyl compounds have two principal UV transitions, the allowed p U p * transition and the forbidden n U p* transition. Allowed 190 nm π Forbidden 280 nm n π

18 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy f these, only the n U p * transition, although it is weak (forbidden), is commonly observed above the usual cutoff points of solvents. Substitution on the carbonyl group by an auxochrome with a lone pair of electrons, such as INR 2, I, IR, IN 2, or IX, as in amides, acids, esters, or acid chlorides, gives a pronounced hypsochromic effect on the n U p * transition and a lesser, bathochromic effect on the p U p * transition. Such bathochromic shifts are caused by resonance interaction similar to that discussed in Section 7.9. Seldom, however, are these effects large enough to bring the p U p * band into the region above the solvent cutoff point. Table 7.6 lists the hypsochromic effects of an acetyl group on the n U p * transition. The hypsochromic shift of the n U p * is due primarily to the inductive effect of the oxygen, nitrogen, or halogen atoms. They withdraw electrons from the carbonyl carbon, causing the lone pair of electrons on oxygen to be held more firmly than they would be in the absence of the inductive effect. If the carbonyl group is part of a conjugated system of double bonds, both the n U p * and the p U p * bands are shifted to longer wavelengths. owever, the energy of the n U p * transition does not decrease as rapidly as that of the p U p * band, which is more intense. If the conjugated chain becomes long enough, the n U p * band is buried under the more intense p U p * band. Figure 7.15 illustrates this effect for a series of polyene aldehydes. Figure 7.16 shows the molecular orbitals of a simple enone system, along with those of the noninteracting double bond and the carbonyl group. TABLE 7.6 YPSRMI EFFETS F LNE-PAIR AUXRMES N TE n U p * TRANSITIN F A ARBNYL GRUP l max e max Solvent nm 12 exane exane 3 l exane 3 N Water Water Ethanol

19 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page 399 FIGURE 7.15 The spectra of a series of polyene aldehydes. (From Murrell, J. N., The Theory of the Electronic Spectra of rganic Molecules, Methuen and o., Ltd., London, Reprinted by permission.) π* 165 nm π 320 nm 218 nm n Alkene Enone arbonyl FIGURE 7.16 The orbitals of an enone system compared to those of the noninteracting chromophores. 4 * 3 * 2 1 π* n 190 nm π 280 nm

20 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy 7.12 WDWARD S RULES FR ENNES The conjugation of a double bond with a carbonyl group leads to intense absorption (e = 8,000 to 20,000) corresponding to a p U p * transition of the carbonyl group. The absorption is found between 220 and 250 nm in simple enones. The n U p * transition is much less intense (e = 50 to 100) and appears at 310 to 330 nm. Although the p U p * transition is affected in predictable fashion by structural modifications of the chromophore, the n U p * transition does not exhibit such predictable behavior. Woodward examined the ultraviolet spectra of numerous enones and devised a set of empirical rules that enable us to predict the wavelength at which the p U p * transition occurs in an unknown enone. Table 7.7 summarizes these rules. TABLE 7.7 EMPIRIAL RULES FR ENNES b b a Base values: Six-membered ring or acyclic parent enone Five-membered ring parent enone Acyclic dienone Increments for: = 215 nm = 202 nm = 245 nm Double-bond-extending conjugation 30 Alkyl group or ring residue a 10 Polar groupings: b 12 g and higher 18 I a 35 b 30 d 50 I 3 a,b,d 6 I 3 a 35 b 30 g 17 d 31 Il a 15 b 12 IBr a 25 b 30 INR 2 b 95 Exocyclic double bond 5 omocyclic diene component 39 Solvent correction d d g b a Variable l Et max (calc) = Total

21 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Woodward s Rules for Enones 401 Following are a few sample applications of these rules. The pertinent parts of the structures are shown in bold face. 3 β α Acyclic enone: 215 nm α - 3 : 10 β - 3 : 2 12 = nm bserved: nm Five-membered enone: 202 nm β -Ring residue: 2 12 = 24 Exocyclic double bond: nm bserved: δ α γ β 226 nm Six-membered enone: 215 nm Double-bond-extending conjugation: 30 β -Ring residue: 12 δ -Ring residue: 18 Exocyclic double bond: 5 bserved: 3 β α 3 R α β γ δ 3 3 Six-membered enone: 215 nm Double-bond-extending conjugation: 30 omocyclic diene: 39 δ-ring residue: nm bserved: α 3 Br Five-membered enone: 202 nm α -Br: 25 β-ring residue: 2 12 = 24 Exocyclic double bond: nm bserved: 280 nm 280 nm β 251 nm 300 nm

22 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy 7.13 a,b-unsaturated ALDEYDES, AIDS, AND ESTERS a,b-unsaturated aldehydes generally follow the same rules as enones (see the preceding section) except that their absorptions are displaced by about 5 to 8 nm toward shorter wavelength than those of the corresponding ketones. Table 7.8 lists the empirical rules for unsaturated aldehydes. Nielsen developed a set of rules for a,b-unsaturated acids and esters that are similar to those for enones (Table 7.9). onsider 2-cyclohexenoic and 2-cycloheptenoic acids as examples: TABLE 7.8 EMPIRIAL RULES FR UNSATURATED ALDEYDES Parent 7.14 ARMATI MPUNDS β β With a or b alkyl groups 220 With a,b or b,b alkyl groups 230 With a,b,b alkyl groups 242 α α, β-dialkyl 217 nm calc. Double bond is in a six-membered ring, adds nothing 217 nm obs. α, β -dialkyl 217 nm Double bond is in a seven-membered ring nm 222 nm calc. 222 nm obs. The absorptions that result from transitions within the benzene chromophore can be quite complex. The ultraviolet spectrum contains three absorption bands, which sometimes contain a great deal of fine structure. The electronic transitions are basically of the p U p * type, but their details are not as simple as in the cases of the classes of chromophores described in earlier sections of this chapter. Figure 7.17a shows the molecular orbitals of benzene. If you were to attempt a simple explanation for the electronic transitions in benzene, you would conclude that there are four possible transitions, but each transition has the same energy. You would predict that the ultraviolet spectrum of benzene consists of one absorption peak. owever, owing to electronelectron repulsions and symmetry considerations, the actual energy states from which electronic transitions occur are somewhat modified. Figure 7.17b shows the energy-state levels of benzene. Three electronic transitions take

23 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Aromatic ompounds 403 TABLE 7.9 EMPIRIAL RULES FR UNSATURATED AIDS AND ESTERS Base values for: β β α With a or b alkyl group R With a,b or b,b alkyl groups 217 With a,b,b alkyl groups 225 For an exocyclic a,b double bond For an endocyclic a,b double bond in a five- or seven-membered ring β β α 208 nm Add 5 nm Add 5 nm place to these excited states. Those transitions, which are indicated in Figure 7.17b, are the so-called primary bands at 184 and 202 nm and the secondary (fine-structure) band at 255 nm. Figure 7.18 is the spectrum of benzene. f the primary bands, the 184-nm band (the second primary band) has a molar absorptivity of 47,000. It is an allowed transition. Nevertheless, this transition is not observed under usual experimental conditions because absorptions at this wavelength are in the vacuum ultraviolet region of the spectrum, beyond the range of most commercial instruments. In polycyclic aromatic compounds, the second primary band is often shifted to longer wavelengths, in which case π 6 * π 4 * π 5 * π 2 π nm (forbidden) E 1u 180 nm (allowed) 260 nm (forbidden) B 1u B 2u π 1 A 1g (a) Molecular orbitals (b) Energy states FIGURE 7.17 Molecular orbitals and energy states for benzene.

24 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy FIGURE 7.18 Ultraviolet spectrum of benzene. (From Petruska, J., Journal of hemical Physics, 34 (1961): Reprinted by permission.) it can be observed under ordinary conditions. The 202-nm band is much less intense (e = 7400), and it corresponds to a forbidden transition. The secondary band is the least intense of the benzene bands (e = 230). It also corresponds to a symmetry-forbidden electronic transition. The secondary band, caused by interaction of the electronic energy levels with vibrational modes, appears with a great deal of fine structure. This fine structure is lost if the spectrum of benzene is determined in a polar solvent or if a single functional group is substituted onto the benzene ring. In such cases, the secondary band appears as a broad peak, lacking in any interesting detail. Substitution on the benzene ring can cause bathochromic and hyperchromic shifts. Unfortunately, these shifts are difficult to predict. onsequently, it is impossible to formulate empirical rules to predict the spectra of aromatic substances as was done for dienes, enones, and the other classes of compounds discussed earlier in this chapter. You may gain a qualitative understanding of the effects of substitution by classifying substituents into groups. A. Substituents with Unshared Electrons Substituents that carry nonbonding electrons (n electrons) can cause shifts in the primary and secondary absorption bands. The nonbonding electrons can increase the length of the p system through resonance Y Y Y Y

25 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Aromatic ompounds 405 The more available these n electrons are for interaction with the p system of the aromatic ring, the greater the shifts will be. Examples of groups with n electrons are the amino, hydroxyl, and methoxy groups, as well as the halogens. Interactions of this type between the n and p electrons usually cause shifts in the primary and secondary benzene absorption bands to longer wavelength (extended conjugation). In addition, the presence of n electrons in these compounds gives the possibility of n U p* transitions. If an n electron is excited into the extended p* chromophore, the atom from which it was removed becomes electron deficient, while the p system of the aromatic ring (which also includes atom Y) acquires an extra electron. This causes a separation of charge in the molecule and is generally represented as regular resonance, as was shown earlier. owever, the extra electron in the ring is actually in a p* orbital and would be better represented by structures of the following type, with the asterisk representing the excited electron: * * Y Y * Y + Y Such an excited state is often called a charge-transfer or an electron-transfer excited state. In compounds that are acids or bases, p changes can have very significant effects on the positions of the primary and secondary bands. Table 7.10 illustrates the effects of changing the p of the solution on the absorption bands of various substituted benzenes. In going from benzene to phenol, notice the shift from to nm a 7-nm shift in the primary band. The secondary band shifts from 254 to 270 nm a 16-nm shift. owever, in phenoxide ion, the conjugate base of phenol, the primary band shifts from to 235 nm (a 31.5-nm shift), and the secondary band shifts from 254 to 287 nm (a 33-nm shift). The intensity of the secondary band also increases. In phenoxide ion, there are more n electrons, and they are more available for interaction with the aromatic p system than in phenol. TABLE 7.10 p EFFETS N ABSRPTIN BANDS Primary Secondary Substituent l (nm) e l (nm) e , I , ,450 I 235 9, ,600 IN , ,430 IN , I , I 224 8, *

26 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy The comparison of aniline and anilinium ion illustrates a reverse case. Aniline exhibits shifts similar to those of phenol. From benzene to aniline, the primary band shifts from to 230 nm (a 26.5-nm shift), and the secondary band shifts from 254 to 280 nm (a 26-nm shift). owever, these large shifts are not observed in the case of anilinium ion, the conjugate acid of aniline. For anilinium ion, the primary and secondary bands do not shift at all. The quaternary nitrogen of anilinium ion has no unshared pairs of electrons to interact with the benzene p system. onsequently, the spectrum of anilinium ion is almost identical to that of benzene. B. Substituents apable of p-onjugation Substituents that are themselves chromophores usually contain p electrons. Just as in the case of n electrons, interaction of the benzene-ring electrons and the p electrons of the substituent can produce a new electron transfer band. At times, this new band may be so intense as to obscure the secondary band of the benzene system. Notice that this interaction induces the opposite polarity; the ring becomes electron deficient. R + R Table 7.10 demonstrates the effect of acidity or basicity of the solution on such a chromophoric substituent group. In the case of benzoic acid, the primary and secondary bands are shifted substantially from those noted for benzene. owever, the magnitudes of the shifts are somewhat smaller in the case of benzoate ion, the conjugate base of benzoic acid. The intensities of the peaks are lower than for benzoic acid as well. We expect electron transfer of the sort just shown to be less likely when the functional group already bears a negative charge.. Electron-Releasing and Electron-Withdrawing Effects Substituents may have differing effects on the positions of absorption maxima, depending on whether they are electron releasing or electron withdrawing. Any substituent, regardless of its influence on the electron distribution elsewhere in the aromatic molecule, shifts the primary absorption band to longer wavelength. Electron-withdrawing groups have essentially no effect on the position of the secondary absorption band unless, of course, the electron-withdrawing group is also capable of acting as a chromophore. owever, electron-releasing groups increase both the wavelength and the intensity of the secondary absorption band. Table 7.11 summarizes these effects, with electronreleasing and electron-withdrawing substituents grouped together. + D. Disubstituted Benzene Derivatives With disubstituted benzene derivatives, it is necessary to consider the effect of each of the two substituents. For para-disubstituted benzenes, two possibilities exist. If both groups are electron releasing or if they are both electron withdrawing, they exert effects similar to those observed with monosubstituted benzenes. The group with the stronger effect determines the extent of shifting of R + R

27 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Aromatic ompounds 407 TABLE 7.11 ULTRAVILET MAXIMA FR VARIUS ARMATI MPUNDS Primary Secondary Substituent l (nm) e l (nm) e Electronreleasing substituents Electronwithdrawing substituents , I , Il , IBr 210 7, I , ,450 I , ,480 IN , ,430 IN , ,000 I , I ,800 I ,400 IN ,800 the primary absorption band. If one of the groups is electron releasing while the other is electron withdrawing, the magnitude of the shift of the primary band is greater than the sum of the shifts due to the individual groups. The enhanced shifting is due to resonance interactions of the following type: 2 N + N 2 N + + N If the two groups of a disubstituted benzene derivative are either ortho or meta to each other, the magnitude of the observed shift is approximately equal to the sum of the shifts caused by the individual groups. With substitution of these types, there is no opportunity for the kind of direct resonance interaction between substituent groups that is observed with para substituents. In the case of ortho substituents, the steric inability of both groups to achieve coplanarity inhibits resonance. For the special case of substituted benzoyl derivatives, an empirical correlation of structure with the observed position of the primary absorption band has been developed (Table 7.12). It provides a means of estimating the position of the primary band for benzoyl derivatives within about 5 nm.

28 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Ultraviolet Spectroscopy TABLE 7.12 EMPIRIAL RULES FR BENZYL DERIVATIVES Parent chromophore: R = alkyl or ring residue 246 R = 250 R = or alkyl 230 Increment for each substituent: Alkyl or ring residue o, m 3 p 10, 3, or alkyl o, m 7 p 25 o 11 m 20 p 78 l o, m 0 p 10 Br o, m 2 p 15 N 2 o, m 13 p 58 N 3 o, m 20 p 45 N 3 p 73 N( 3 ) 2 o, m 20 p 85 Following are two sample applications of these rules: Br Parent chromophore: 246 nm o-ring residue: 3 m-br: nm bserved: 253 nm R Parent chromophore: 230 nm m-: 2 7 = 14 p-: nm bserved: 270 nm

29 14782_07_h7_p pp2.qxd 2/2/08 1:23 AM Page Aromatic ompounds 409 E. Polynuclear Aromatic ydrocarbons and eterocyclic ompounds Researchers have observed that the primary and secondary bands in the spectra of polynuclear aromatic hydrocarbons shift to longer wavelength. In fact, even the second primary band, which appears at 184 nm for benzene, is shifted to a wavelength within the range of most UV spectrophotometers. This band lies at 220 nm in the spectrum of naphthalene. As the extent of conjugation increases, the magnitude of the bathochromic shift also increases. The ultraviolet spectra of the polynuclear aromatic hydrocarbons possess characteristic shapes and fine structure. In the study of spectra of substituted polynuclear aromatic derivatives, it is common practice to compare them with the spectrum of the unsubstituted hydrocarbon. The nature of the chromophore can be identified on the basis of similarity of peak shapes and fine structure. This technique involves the use of model compounds. Section 7.15 will discuss it further. Figure 7.19 shows the ultraviolet spectra of naphthalene and anthracene. Notice the characteristic shape and fine structure of each spectrum, as well as the effect of increased conjugation on the positions of the absorption maxima. FIGURE 7.19 Ultraviolet spectra of naphthalene and anthracene. (From Friedel, R. A., and M. rchin, Ultraviolet Spectra of Aromatic ompounds, John Wiley and Sons, New York, Reprinted by permission.)